Actually, they've developed ultralight water. Regular water is of course H2O. They out the oxygen to save weight, leaving only hydrogen behind, which as we all know is lighter than air. Thus, the more "de-oxygenated" water you carry, the lighter your pack. When you get to camp and need water, all you have to do is to breathe into a special filter which reoxygenates the water. Brilliant in it's simplicity, really.